In mobile radio communication system, a plurality of base stations (eNodeB) are arranged so as to cover a communication area like cells, forming a cellular structure to thereby making it possible to enlarge the communication area (which is called a cellular system). A mobile station (mobile terminal, UE (User Equipment)) usually selects one base station which is good in communication quality (channel condition) and connects to that base station.
In uplink, it is necessary for a base station to make the arrival times of the data signals transmitted from a plurality of mobile stations that have selected to connect to the base station per se, put within a predetermined time range. For example, in a mobile radio communication system such as LTE (Long Term Evolution), LTE-Advanced using OFDMA (Orthogonal Frequency Division Multiple Access), SC-FDMA (Single Carrier-Frequency Division Multiple Access), DFT-spread OFDM (Discrete Fourier Transform-spread Orthogonal Frequency Division Multiplexing) or DFT-precoded OFDM (Discrete Fourier Transform-precoded Orthogonal Frequency Division Multiplexing), the time differences in arrival of data signals from different mobile stations at the base station may be made to fall within the length of CP (Cyclic Prefix) to thereby suppress inter-symbol interference and inter-carrier interference due to time difference of arrival.
The CP is a guard interval that is added to precede the valid symbol in order to avoid influence of multipath fading in a multicarrier transmission such as OFDM transmission, and is a guard interval that is added to precede the OFDMA symbol or SC-FDMA symbol in the aforementioned OFDMA and SC-FDMA.
In these communication schemes, multiple access between mobile stations can be carried out based on sections (e.g., resource blocks) into which the resource is divided with respect to the frequency domain and time domain. Accordingly, in uplink, the propagation distance of each mobile station to the base station becomes different depending on the relative position between the base station and mobile station.
FIG. 24 shows an example when a mobile station 1000-1 and mobile station 1000-2 make choice of connection to a base station 2000 while mobile station 1000-3 and mobile station 1000-4 make choice of connection to a base station 3000, where t12 is the time of arrival of the signal transmitted by mobile station 1000-1 at base station 2000, t22 is the time of arrival of the signal transmitted by mobile station 1000-2 at base station 2000, t33 is the time of arrival of the signal transmitted by mobile station 1000-3 at base station 3000, and t43 is the time of arrival of the signal transmitted by mobile station 1000-4 at base station 3000.
When the length of the CPs added to the data signals transmitted to base station 2000 by mobile station 1000-1 and 1000-2 is tcp, base station 2000 transmits a control signal (timing adjustment signal, Timing Advance command) that informs the data signal transmission timing satisfying |t12-t22|<tcp, to each mobile station, and each mobile station transmits its data signal to base station 2000, based on that transmission timing.
In this case, it is preferable that the transmission timing of each mobile station is controlled so that the data signals the different mobile station transmit, simultaneously arrive at base station 2000. Here, |x| indicates the absolute value of x.
Similarly, mobile station 1000-3 and mobile station 1000-4 transmit respective data signals to base station 3000 at timings that satisfy |t33-t43|<tcp. This transmission timing control is performed for each mobile station so that the base station can receive data signals transmitted from different mobile stations simultaneously, to thereby avoid interference between mobile stations.
Here, a base station manager 10 is an apparatus that manages base station 2000 and base station 3000, and is connected to the base stations by wired networks, etc. This apparatus has the functions of, for example, control for performing cooperative communication between base stations, handover control and others. Here, base station 2000 and/or base station 3000 may have the function of base station manager 10.
In a cellular system of this kind, it is possible for a mobile station located at the cell-edge area to perform communication without being affected by interference from the adjacent cell, by using different frequencies between adjacent cells (sectors). However, this entails the problem that frequency use efficiency degrades. To deal with this, by making use of an identical frequency iteratively in different cells (sectors) it is possible to sharply improve frequency use efficiency, but it is necessary to take a measure against interference from the adjacent cell with mobile stations located at the cell-edge area. Further, since mobile stations are limited as to transmission power hence the level of power of signals reaching the base station when the mobile station is located at the cell-edge area, is low, the communication results in a low-data rate.
Under such circumstances, methods of mitigating or suppressing interference with mobile stations located at the cell-edge area by performing inter-cell cooperative communication, i.e., cooperation between neighboring cells, and methods of compensating for the power level of arrival signals, have been investigated. As an example of such a scheme, CoPM (Cooperative Multipoint) transmission scheme and the like have been discussed in a non-patent document 1.
FIG. 25 is a diagram showing one example of a CoMP transmission scheme in uplink, in which a mobile station 100-1 located at the cell-edge area is performing cooperative communication. Mobile station 100-1 is a mobile station that performs cooperative communication with a base station 200 and base station 300. Here, t′12 is the time at which the signal transmitted by mobile station 100-1 reaches base station 200 and t′13 is the time at which the signal transmitted by mobile station 100-1 reaches base station 300.
Here, mobile station 100-2 is a mobile station that communicates with base station 200 only (t′22 is the time at which the signal transmitted by mobile station 100-2 reaches base station 200), and mobile station 100-3 is a mobile station that communicates with base station 300 only (t′33 is the time at which the signal transmitted by mobile station 100-3 reaches base station 300).
Mobile station 100-1 transmits the same data signal to both base station 200 and base station 300. Base station 300 transmits the data signal received from mobile station 100-1 to base station 200 by way of a wired line such as an optical fiber or the like (e.g., the X2 interface in LTE) while base station 200 performs a signal detecting process such as a decoding process and the like, using the data signal directly received from mobile station 100-1 and the data signal of mobile station 100-1 transmitted by way of base station 300.
As a result the data signal transmitted by mobile station 100-1 can be reduced in inter-cell interference and increased in signal power at the time of data signal detection, by resource allocation scheduling and site diversity effect based on the traffic conditions (cell environment) of both base station 200 and base station 300, hence making it possible to improve the transmission characteristics of the mobile station located at the cell-edge area.
Meanwhile, the base station that performs various sorts of controls for communication on a mobile station that is transmitting a data signal to a plurality of base stations is called an anchor base station whereas the base stations other than this are called cooperative base stations. Here, the anchor base station may be defined as a base station that transmits downlink control signals (DCI: Downlink Control Information) through the PDCCH (Physical Downlink Control CHannel).
However, in the uplink inter-cell cooperative communication as above, there occur cases in which |t′12-t′22|<tcp and |t′13-t′33|<tcp cannot hold simultaneously, due to difference between the channel condition between mobile station 100-1 and base station 200 and the channel condition between mobile station 100-1 and base station 300.
For example, when the timing of data signal transmission from mobile station 100-1 is set based on the channel condition between mobile station 100-1 and base station 200, it is possible to make the time difference of arrival between mobile station 100-1 and mobile station 100-2, both connecting to base station 200, fall within a time range equal to or shorter than the CP length. However, since the above transmission timing is not set by taking the channel condition between mobile station 100-1 and base station 300 into consideration, there occur cases where the time difference of arrival at base station 300 between the data signal from mobile station 100-1 and the data signal from mobile station 100-3 is equal to or greater than the CP length. As a result, in OFDM transmission, there has been the problem of characteristics degradation due to inter-symbol interference and inter-carrier interference. Further, in SC-FDMA transmission, there occur characteristics degradation attributed to collapse of the periodicity of the FFT (DFT) and characteristics degradation attributed to interference between signals in the FFT duration (inter block interference).
To deal with this problem, in order to avoid the problem as to the above delay, non-patent document 2 discloses a new transmission timing control method on the basis of cooperative communication and a method of making the CP length longer. With this scheme, the time difference of arrival is made to fall within the CP length to thereby suppress interference.